Metal-clad electric gear insulated by gas under pressure, e.g. sulphur hexafluoride, must remain hermetically sealed for many months and, even, years, without it being necessary to top up the quantity of gas. One technique is to use two seals placed in series along a leakage path. This considerably reduces the danger of leakage. Another technique is to check whether the most interior seals of a unit are really effective by measuring the leaks in the volume situated between the two seals. This is done by providing a hole in the flange which hole communicates firstly with the outside and secondly in the zone situated between the two seals, the outside orifice being normally closed and sealed by a bolt which clamps a seal.
Preferred embodiments of the invention provide apparatus for checking for leaks between components of metal-clad electric gear containing gas under pressure which checking apparatus requires only simplified machining and whose price is consequently competitive.